CMOS image sensors are increasingly being used as low cost imaging devices. A CMOS image sensor circuit includes a focal plane array of pixel cells, each one of the cells includes a photosensor, such as e.g., a photogate, photoconductor, or photodiode having an associated charge accumulation region within a substrate for accumulating photo-generated charge. Each pixel cell may include a transistor for transferring charge from the charge accumulation region to a sensing node, and a transistor for resetting the sensing node to a predetermined charge level prior to charge transference. The pixel cell may also include a source follower transistor for receiving and amplifying charge from the sensing node and an access transistor, for controlling the readout of the cell contents from the source follower transistor.
In a CMOS image sensor, the active elements of a pixel cell perform the necessary functions of: (1) photon to charge conversion; (2) accumulation of image charge; (3) transfer of charge to the sensing node accompanied by charge amplification; (4) resetting the sensing node to a known state before the transfer of charge to it; (5) selection of a pixel for readout; and (6) output and amplification of a signal representing pixel charge from the sensing node.
CMOS image sensors of the type discussed above are generally known as discussed, for example, in Nixon et al., “256×256 CMOS Active Pixel Sensor Camera-on-a-Chip,” IEEE Journal of Solid-State Circuits, Vol. 31(12), pp. 2046-2050 (1996); and Mendis et al., “CMOS Active Pixel Image Sensors,” IEEE Transactions on Electron Devices, Vol. 41(3), pp. 452-453 (1994). See also U.S. Pat. Nos. 6,177,333 and 6,204,524, which describe the operation of conventional CMOS image sensors and are assigned to Micron Technology, Inc., the contents of which are incorporated herein by reference.
A top-down view of a conventional CMOS pixel cell 10 is shown in FIG. 1. The illustrated CMOS pixel cell 10 is a four transistor (4T) cell. The CMOS pixel cell 10 generally comprises a photo-conversion device 13, e.g., a photodiode, for generating and collecting charge in response to light incident on the pixel cell 10, and a transfer transistor having a gate 7 for transferring photoelectric charges from the photo-conversion device 13 to a sensing node, which is typically a floating diffusion region 3. The floating diffusion region 3 is electrically connected to the gate 27 of an output source follower transistor. The pixel cell 10 also includes a reset transistor having a gate 17 for resetting the floating diffusion region 3 to a predetermined voltage; and a row select transistor having a gate 37 for outputting a signal from the source follower transistor 27 to an output terminal in response to an address signal on gate 37.
FIG. 2 is a cross-sectional view of a portion of the pixel cell 10 of FIG. 1, taken along line 2-2′, showing the photo-conversion device 13 constructed as a photodiode, transfer transistor having a gate 7 and reset transistor having a gate 17. The CMOS pixel cell 10 has a photo-conversion device 13 that may be formed as a pinned photodiode. The illustrated photodiode has a p-n-p construction comprising a p-type surface layer 5 and an n-type photodiode charge collection region 14 within a p-type substrate 2. The photodiode 13 is adjacent to and partially underneath the gate 7 of the transfer transistor. The reset transistor gate 17 is on a side of the transfer transistor gate 7 opposite the photodiode 13. As shown in FIG. 2, the reset transistor includes a source/drain region 32, which is adjacent an isolation region 9. The floating diffusion region 3 is located between the gates 7, 17 of the transfer and reset transistor.
In the CMOS pixel cell 10 depicted in FIGS. 1 and 2, electrons are generated by light incident on the photodiode 13 and are stored in the n-type photodiode region 14. These charges are transferred to the floating diffusion region 3 by the transfer transistor when the transfer transistor gate 7 is turned on. The source follower transistor produces an output signal from the transferred charges which are stored in the floating diffusion region 3.
One common problem associated with conventional imager pixel cells, such as pixel cell 10, is dark current, that is, current generated as a photodiode signal in the absence of light. Dark current may be caused by many different factors, including: photodiode junction leakage, leakage along isolation edges, transistor sub-threshold leakage, drain induced barrier lower leakage, gate induced drain leakage, trap assisted tunneling, and pixel fabrication defects. One example of a defect is an interstitial vacancy state in the charge carrier-depletion region. This defect causes increased thermal generation of electron-hole pairs, which may be collected in the photodiode 13 (FIG. 2) and effectively lower overall image quality.
The area directly under the edge of the transfer transistor gatestack 17 is a significant source of dark current. The n-type accumulation region 14 of photodiode 13 is formed close to the surface of the substrate 2 under the transfer gatestack 17 in order to reduce charge lag. This causes the depletion region created during an integration period for the pixel cell 10, and being associated with the n-type accumulation region 14 and the p-type surface region 5, to also be close to the surface of the substrate 2 in this area. The presence of the depletion region in an area that already has defects causes large numbers of thermally-created electron-hole pairs to be present in this area near the transfer transistor gatestack 17 edge. When the photodiode 13 is reset with a reset voltage applied on the reset gate 17, a reverse bias electric field sweeps the thermally created holes into the p-type surface region 5 and the thermally created charge carriers over to the n-type collection area 14 of the photodiode 13. These thermally generated charge carriers increase the unwanted dark current for image pixel cell 10.
One possible solution to reducing the dark current generation underneath the transfer transistor gatestack is to apply a negative voltage on the transfer transistor's gate. The negative voltage attracts electron-hole pairs to the surface, decreasing the depletion region there and effectively covering the interstitial vacancy state. Accordingly, with a negative voltage applied to the transfer transistor gate, thermally generated electron-hole pairs will likely recombine before the photodiode can collect them. This solution, however, tends to aggravate another problem, referred to as blooming. Blooming occurs when the storage capacity of the photodiode is full and electrons are still being generated even though the photodiode is full. The extra electrons can bloom to several locations. The extra electrons may attempt to diffuse by jumping across isolation barriers into adjacent pixels, corrupting their signals. Alternatively, the electrons may travel through the substrate and be collected in other areas of the pixel or in periphery circuit devices. The floating diffusion region is the most desirable place for the extra electrons to be collected. The floating diffusion region has considerable capacity to store these stray electrons during imager operation and the signal on the floating diffusion region is cleared or reset before the pixel signal is actually read.
Positively biasing the transfer transistor gate 7 makes extra electrons more likely to bloom through the transfer transistor to the floating diffusion region 3. However, applying a negative bias to the transfer transistor gate 7, which is desirable to prevent dark current penetration, makes it more difficult for the extra electrons to bloom to the floating diffusion region 3, thus causing blooming into other undesirable regions of a pixel or adjacent pixels. Moreover, as suggested above, a positively biased transfer transistor gate 7 increases the dark current as a result of a larger depletion region under the transfer transistor gate 7.
Therefore, a pixel having a decreased dark current without negative blooming effects is desired. Also needed is a simple method of fabricating and operating such a pixel.